Nuclear magnetic resonance spectroscopy (NMR) provides a method of noninvasively monitoring flux through the various pathways of energy metabolism. This is accomplished by examining the kinetics of 13C transfer from a suitable, 13C labeled substrate, and fitting data to an appropriate metabolic network model. In 1983, Chance et al. developed such a model for determining flux through the tricarboxylic acid (TCA) cycle of the heart from the kinetics of 13C glutamate labeling. We recently validated this model by comparing rates of oxygen comsumption by Langendorff perfused rat hearts with oxygen comsumption rates predicted from the model, glucose, acetate and b-hydroxybutyrate were substrates. The results were in excellent agreement after the original model was modified to include the malate-aspartate and glycerol-3-phosphate shuttles. We now propose to extend this model to incorporate other pathways of energy metabolism and to apply this model to other tissues such as tumo r, skeletal muscle, brain and liver.